Early "Puberty"?

I posted this in raising chicks forum as well. But I thought I would post here, too, & see if anyone has an answers for me. I have 24 chicks a mix of Golden Comets, Arucana, & Black Astralorps. They range between 5-6wks old. I have 2 roos (Astralorps). Today, the larger of the the roos was attempting to mate with one of my hens. This seems way early to me. Am I right? If so, do I need to be concerned? So far, other than the girls not being thrilled about it, I think everyone is fine.